5-45 Chapter 5—Processor Design—Advanced Topics Microprogramming: Basic Idea • Recall control sequence for 1-bus SRC Step Concrete RTN Control Sequence T0 MA ← PC: C ← PC + 4; PCout, MAin, INC4, Cin, Read ← ← T1 MD M[MA]: PC C; Cout, PCin, Wait T2 IR ← MD; MDout, IRin ← T3 A R[rb]; Grb, Rout, Ain ← T4 C A + R[rc]; Grc, Rout, ADD, Cin ← T5 R[ra] C; Cout, Gra, Rin, End • Control unit job is to generate the sequence of control signals • How about building a computer to do this? Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-46 Chapter 5—Processor Design—Advanced Topics The Microcode Engine • A computer to generate control signals is much simpler than an ordinary computer • At the simplest, it just reads the control signals in order from a read-only memory • The memory is called the control store • A control store word, or microinstruction, contains a bit pattern telling which control signals are true in a specific step • The major issue is determining the order in which microinstructions are read Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-47 Chapter 5—Processor Design—Advanced Topics Fig 5.16 Block Diagram of Microcoded Control Unit Ck CCs Other IR Opcode PLA • Microinstruction has Sequencer (computes branch control, 2 start addr) External source n branch address, and control signal fields Increment 4–1 Mux n • Microprogram µPC counter can be set n from several sources to do the required Control sequencing store k n m µBranch µIR control Branch Control signals address PCout, etc. Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-48 Chapter 5—Processor Design—Advanced Topics Parts of the Microprogrammed Control Unit • Since the control signals are just read from memory, the main function is sequencing • This is reflected in the several ways the µPC can be loaded • Output of incrementer—µPC + 1 • PLA output—start address for a macroinstruction • Branch address from µinstruction • External source—say for exception or reset • Micro conditional branches can depend on condition codes, data path state, external signals, etc. Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-49 Chapter 5—Processor Design—Advanced Topics Contents of a Microinstruction Microinstruction format Branch control Control signals Branch address in in out out in PC C A End MA PC • Main component is list of 1/0 control signal values • There is a branch address in the control store • There are branch control bits to determine when to use the branch address and when to use µPC + 1 Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-50 Chapter 5—Processor Design—Advanced Topics Fig 5.17 The Control Store 0 µCode for instruction fetch • Common instruction a1 µCode for add fetch Microaddress sequence • Separate a2 µCode for br sequences for each (macro) instruction a3 µ Code for shr • Wide words 2n-1 m bits wide k µbranch c control n branch control bits signals addr. bits Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-51 Chapter 5—Processor Design—Advanced Topics Tbl 5.2 Control Signals for the . add Instruction 101 ••• 100011000011000000 102 ••• 010000100000100000 103 ••• 001000010000000000 200 ••• 0 0010000 1 000000 1 00 201 ••• 0 0000000011000 1 00 1 202 ••• 0 100000001000011 00 • Addresses 101–103 are the instruction fetch • Addresses 200–202 do the add • Change of µcontrol from 103 to 200 uses a kind of µbranch Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-52 Chapter 5—Processor Design—Advanced Topics Uses for µbranching in the Microprogrammed Control Unit (1) Branch to start of µcode for a specific inst. → (2) Conditional control signals, e.g. CON PCin (3) Looping on conditions, e.g. n ≠ 0 → ... Goto6 • Conditions will control µbranches instead of being ANDed with control signals • Microbranches are frequent and control store addresses are short, so it is reasonable to have a µbranch address field in every µ instruction Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-53 Chapter 5—Processor Design—Advanced Topics Illustration of µbranching Control Logic • We illustrate a µbranching control scheme by a machine having condition code bits N and Z • Branch control has 2 parts: (1) selecting the input applied to the µPC and (2) specifying whether this input or µPC + 1 is used • We allow 4 possible inputs to µPC • The incremented value µPC + 1 • The PLA lookup table for the start of a macroinstruction • An externally supplied address • The branch address field in the µinstruction word Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-54 Chapter 5—Processor Design—Advanced Topics Fig 5.18 Branching Controls in the Microcoded Control Unit ZN Sequencer PLA • 5 branch 2 External address conditions 2 • NotN 2 2 4–1 Mux • N 2 • NotZ Incr. µPC 2 Branch address • Z • Unconditional Control store • To 1 of 4 places • Next 0 0 0 0 0 0 0 Control signals 244 10 µinstruction 2 Mux control • PLA BrUn BrNotZ Mux Ctl Select • External BrZ 00 Increment µPc BrNotN 01 PLA address BrN 10 External address 11 Branch address • Branch address Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-55 Chapter 5—Processor Design—Advanced Topics Some Possible µbranches Using the Illustrated Logic (Refer to Tbl 5.3) . Cont r ol Branch Signals Address Branching act ion 00 0 0000 ••• XXX None—next instruct ion 01 1 0000 ••• XXX Branch to output of PLA 10 00100 ••• XXX Br if Z to Extern. Addr. 11 00001 ••• 300 Br if N to 300 (else next) 11 0001 0 0• • • 0 206 Br if N to 206 (else next) 11 1 0000 ••• 204 Br t o 20 4 • If the control signals are all zero, the µinstruction only does a test • Otherwise test is combined with data path activity Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-56 Chapter 5—Processor Design—Advanced Topics Horizontal versus Vertical Microcode Schemes • In horizontal microcode, each control signal is represented by a bit in the µinstruction • In vertical microcode, a set of true control signals is represented by a shorter code • The name horizontal implies fewer control store words of more bits per word • Vertical µcode only allows RTs in a step for which there is a vertical µinstruction code • Thus vertical µcode may take more control store words of fewer bits Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-57 Chapter 5—Processor Design—Advanced Topics Fig 5.19 A Somewhat Vertical Encoding ALU Register-out ops field field µIR F5 F8 4 3 4–16 decoder 3–8 decoder 16 ALU 7 Regout control control signals signals • Scheme would save (16 + 7) - (4 + 3) = 16 bits/word in the case illustrated Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-58 Chapter 5—Processor Design—Advanced Topics Fig 5.20 Completely Horizontal and Vertical Microcoding µPC Vertical control Horizontal store µPC control store PC C Inc4 MA Data in out n to 2n decoder path in PCout MAin Inc4 Cin Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-59 Chapter 5—Processor Design—Advanced Topics Saving Control Store Bits with Horizontal Microcode • Some control signals cannot possibly be true at the same time • One and only one ALU function can be selected • Only one register out gate can be true with a single bus • Memory read and write cannot be true at the same step • A set of m such signals can be encoded using log2m bits (log2(m + 1) to allow for no signal true) • The raw control signals can then be generated by a k to 2k decoder, where 2k ≥ m (or 2k ≥ m + 1) • This is a compromise between horizontal and vertical encoding Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-60 Chapter 5—Processor Design—Advanced Topics A Microprogrammed Control Unit for the 1-Bus SRC • Using the 1-bus SRC data path design gives a specific set of control signals • There are no condition codes, but data path signals CON and n = 0 will need to be tested • We will use µbranches BrCON, Brn = 0, and Brn ≠ 0 • We adopt the clocking logic of Fig. 4.14 • Logic for exception and reset signals is added to the microcode sequencer logic • Exception and reset are assumed to have been synchronized to the clock Computer Systems Design and Architecture by V. Heuring and H. Jordan © 1997 V. Heuring and H. Jordan 5-61 Chapter 5—Processor Design—Advanced Topics Tbl 5.4 The add Instruction .. Ot her Br Addr. Cont ro l Actions Addr. Signals 100 000000011 • • • XXX MA ← PC: C ← PC+4 ; 101 000000000 • • • XXX MD ← M[ MA] : PC ← C; • • • 102 011000000 XXX I R ← MD; µPC ← PLA ; • • • 200 00 0000000 XXX A ← R [rb ]; 201 00 0000000 • • • XXX C ← A + R[rc]; • • • 202 11 110000 0 100 R [ra] ← C: µPC ← 100; • Microbranching to the output of the PLA is shown at 102 • Microbranch to 100 at 202 starts next fetch Computer Systems Design and Architecture by V.